[DRBD-user] drbd or rsync?

Greg Freemyer greg.freemyer at gmail.com
Wed Apr 8 20:41:36 CEST 2009


Adding linux-ha to cc list.

The discussion is about using the drbd secondary as a source for
nightly backups.

On Wed, Apr 8, 2009 at 2:19 PM, Lars Ellenberg
<lars.ellenberg at linbit.com> wrote:
> On Wed, Apr 08, 2009 at 01:34:43PM -0400, Greg Freemyer wrote:
>> On Wed, Apr 8, 2009 at 1:13 PM, Peter Sabaini <peter at sabaini.at> wrote:
> >>
>> >> Anyhow, provided the bandwidth is sufficient, I prefer the DRBD
>> >> option.  However, will the backups be successful, if the backup drive
>> >> is attached to the secondary?  Most of what I've read suggests that
>> >> you shouldn't even mount the secondary in read-only mode (although
>> >> maybe an LVM snapshot can be mounted for taking a backup).
>> >
>> > DRBD won't allow you to mount a secondary. This means you either have to use a
>> > Dual Primary setup (requiring a cluster filesystem) or make a block-level
>> > backup (don't know if thats adivsable). Maybe you can temporarily disconnect
>> > DRBD for the backup, promote the secondary, mount + backup, demote again, and
>> > reconnect -- Im not really sure if this would work though, you'd have to try.
>> >
>> > peter.
>>
>> I also don't think a snapshot will work either.  The first step in
>> creating a snapshot is to quiesce every thing, then create the
>> snapshot.  Since the quiesce would have to happen on the primary, I
>> don't think you could properly coordinate a snapshot being created on
>> the secondary.
>>
>> If tape is critical to your situation, you may need to stick to rsync.
>>
>> FYI: I think this should be added to the drdb wishlist.  I think some
>> commercial SAN devices that replicate have to ability to mount the
>> remote copy by using snapshot technology on the remote.
>>
>> And I know some support what Peter describes which is effectively:
>>
>> quiesce primary
>
> xfs_freeze -f  # or pick your favorite equivalent
>
> but, you have to be aware that "quiesce filesystem" (which is what a
> local lvm snapshot would implicitly try to do for you on recent Linux
> OS), is _not_ the same as "quiesce primary".
> you need to actually "quiesce" all applications as well...
>
> [*] but see below.
>
>> stop block transmissions from primary to secondary
>
> drbdadm disconnect
>
>> mount secondary in read-only form
>> perform backup
>> unmount secondary
>> release drdb to continue updates
>
> even mount -o ro _does_ rw access to the device in general (update
> superblock, replay journal...).  and drbd does not and will not support
> to mount a device in secondary role.

At a minimum XFS has a specific mount option for true readonly
devices.  The option was put in specifically to support truly readonly
snapshots.  I don't know if any of the other common linux fs'es have
that feature.

> still, you can introduce an arbitrary "split brain" here,
> make it primary, and mount -o ro that one,
> then later connect --discard-my-data, to revert potential (minimal)
> local changes and apply changes done on the remote "real/production"
> primary since the disconnect.
>
>> If drdb doesn't support that, I'd also like it to go on the wishlist.
>
> [*] much easier, and already suggested:
> just do an lvm snapshot below the drbd on secondary,
> and mount that one readonly.
>
> ignore the "quiesce" point, or replace it with a simple "sync;".
> if your file system and applications are able to recover from
> a crash, they are able to recover from a snapshot while taken
> even when not "quiesce"d.
> if they are not able to recover: don't use them.
>
> of course, for any applications supporting "checkpoints",
> it would shorten the necessary recovery time if you
> set a checkpoint on the active node just before
> you take the snapshot on the secondary.

Lars,

I see your feedback above, but you are showing doing some stuff on the
primary, then some stuff on the secondary.

Maybe the drbd admin gui tool could leverage pacemaker / heartbeat to
support this.

If one wants to automate a nightly backup as an example, for most
scenarios it would be:

quiesce primary (yes, quiesce apps, quiesce fs, quiesce drdb secondary)
quiesce drdb secondary ( drbdadm disconnect or possibly something less drastic)
lvm snapshot secondary at direction of primary (via pacemaker / heartbeat)
have secondary mount snapshot (via pacemaker)
start backup (via pacemaker)

Seems like a great feature to add to the drdb admin gui.

Greg
-- 
Greg Freemyer
Head of EDD Tape Extraction and Processing team
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